Naphtha reforming is one well-known method of producing C6 to C8 aromatic compounds and involves contacting an aliphatic and/or naphthenic hydrocarbon mixture, such as C5-205° C. naphtha cut from a crude oil distillation unit, with a reforming catalyst, such as as platinum on alumina. Reforming involves a complex series of chemical reactions, including cracking, dehydrocyclization, dehydrogenation and isomerization, to produce a product mixture containing a wide variety of aromatic compounds, including benzene, toluene, xylenes (ortho-, para- and meta-xylenes), ethylbenzene and heavy aromatics, such as mesitylene, pseudocumene, ethyltoluenes and other C9–C12 aromatics.
Frequently, however, there is a need, particularly in the chemical industry, to selectively produce one or more particular C6 to C8 aromatic compounds, such as the para-xylene (PX) used as an intermediate in the production of polyesters.
U.S. Pat. No. 3,449,461 discloses the production of mixed xylenes and other aromatics by subjecting a paraffinic feed to dehydrocyclization over a sulfided refractory oxide catalyst containing a noble metal such as platinum. In accordance with U.S. Pat. No. 3,766,291, feedstock comprising 3-methylbutene-1 is converted to PX by disproportionation to 2,5-dimethylhexene which is subsequently dehydrocyclized over a catalyst containing at least one Group VIII metal associated with tin in combination with a Group II aluminate spinel support material.
U.S. Pat. No. 3,428,702 discloses the dehydrocyclization of 2,5-dimethylhexene in the presence of H2S using a chromia-alumina catalyst such that 30–40% of the 2,5-dimethylhexene is converted to PX. Other dehydrocyclization processes for converting aliphatic or olefinic hydrocarbons to xylenes are found in U.S. Pat. No. 3,207,801, wherein catalysts based on magnesium oxide, hydoxide or magnesium acid salts are used, and in U.S. Pat. No. 4,910,357 wherein a platinum-loaded, non-acidic, metal modified zeolite support such as ZSM-5 is used.